The present invention relates a position measuring method and a position measuring instrument, by which it is possible to measure a position in GPS (a global positioning system) even when a position measurement by GPS cannot be performed.
In recent years, a position measurement using GPS has been propagated. For instance, when it is planned to carry out a civil engineering operation by using civil engineering and construction machineries such as bulldozer, an operating position, i.e. a position of the bulldozer, is determined by GPS, and the operating position is confirmed. Or, as in a case of a car navigator, electronic map information is linked with positional data obtained by GPS. Then, the present position of a vehicle is reflected on an electronic map and is turned to an image, and this image is displayed on an image display device. Thus, a position of a mobile object can be measured at real time.
However, radio waves from satellite are used in the position measurement by GPS. There are often places (shade) where radio waves cannot reach because radio waves from the satellite are interrupted by obstacles, for instance, mountain or building. Or, like the case of the car navigator, for which a range of position measurement is not clearly determined, there are often shades where radio waves do not reach.
For the range where the position measurement by GPS cannot be carried out, it has been practiced in the past that position measurement had to be continued by a surveying operation of man power by using a conventional type surveying instrument.
In the case of a car navigator etc., which is installed on the mobile object, the position measurement cannot be performed on the portion of the shade, and satisfactory function could not be fulfilled as the position measuring instrument.
In this respect, JP-A-2007-171048 discloses an interpolation method on positional data when the position measurement cannot be carried out by GPS. According to the interpolation method disclosed in JP-A-2007-171048, for the range where the position measurement cannot be performed by GPS, sceneries in surroundings are taken continuously as digital images during a process where the mobile object is moved. Then, tracking points are generated in the digital images thus acquired. The tracking points are traced in the images, which are continuously acquired. The tracking points are sequentially identified on the images acquired. Then, from results of orientation of the tracking points at a first point and at a second point and from positional data at the first point and the second point, which are already known by GPS position measurement, three-dimensional positional data of the tracking point is determined (a method of intersection). Based on a result of orientation of the tracking point acquired at a third point and on three-dimensional positional data of the tracking points, a position of the third point is sequentially determined (a method of resection).
By the positional data interpolation method according to JP-A-2007-171048, it is possible to perform the position measurement in a case where the GPS position measurement cannot be accomplished.
On the other hand, the positional data interpolation method according to JP-A-2007-171048 is based on conditions that continuous images can be acquired, that tracking of points between the images can be achieved, and that image surveying can be carried out. In actual position measuring operation, however, continuous image may not be acquired. For instance, in the position measurement under the condition where buildings are present along a road, if there is a space between buildings or if a building is lacking, the images cannot be acquired on the place where the building is not present, and tracking on the images cannot be achieved. Or, even when the tracking points are obtained in the images, there is no continuity in three-dimensional positional data of the tracking points, or three-dimensional positional data may vary widely. As a result, such trouble may occur that an accuracy of measurement of the position of the third point obtained according to the three-dimensional positional data in the images will be extremely lowered.
Further, there is a property that in procedure to perform the position measurement by repeating the method of intersection and the method of resection, measurement errors may be accumulated. Therefore, when a condition where the GPS position measurement cannot be performed for long time, reliability of the measurement is lowered.